molecular basis of inheritance

• What was a major challenge to biologists in the early 20th century?


  The identification of the molecules of inheritance
• What did T. H. Morgan's group show about genes?


  Genes are located on chromosomes
• What were the two components of chromosomes that became candidates for the genetic material?


  DNA and protein
• Who first discovered the role of DNA in heredity?

  Frederick Griffith
• What phenomenon did Griffith observe when he mixed heat-killed pathogenic bacteria with harmless bacteria?


  Transformation
• How is transformation defined?


  As a change in genotype and phenotype due to assimilation of foreign DNA
• Who identified the transforming substance as DNA?

  Oswald Avery, Maclyn McCarty, and Colin MacLeod
• What are viruses that infect bacteria called?

  Bacteriophages
• What did Hershey and Chase demonstrate in their 1952 experiment?


  That DNA is the genetic material of the phage T2
• What is the structure of DNA composed of?

  A polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group
• What are the four nitrogenous bases found in DNA?


  Adenine (A), thymine (T), guanine (G), and cytosine (C)
• What did Erwin Chargaff report about DNA composition in 1950?


  That DNA composition varies from one species to the next
• What are Chargaff's rules?


  The base composition of DNA varies between species, and in any species, the number of A and T bases is equal, and the number of G and C bases is equal
• What technique did Maurice Wilkins and Rosalind Franklin use to study DNA structure?


  1. ray crystallography
• What did Franklin's X-ray images suggest about the DNA molecule?


  That it was helical and made up of two strands forming a double helix
• How did Watson and Crick's model of DNA differ from their initial assumptions about base pairing?


  They realized that pairing a purine with a pyrimidine resulted in a uniform width
• What does the Watson-Crick model explain about Chargaff's rules?


  That in any organism, the amount of A equals T, and the amount of G equals C
• What is the process of copying DNA called?

  DNA replication
• What does the semiconservative model of replication predict?


  Each daughter molecule will have one old strand and one newly made strand
• What did Meselson and Stahl's experiments support?


  The semiconservative model of DNA replication
• What begins the process of DNA replication?


  Particular sites called origins of replication
• What is a replication fork?


  A Y-shaped region where parental DNA strands are being unwound
• What role do helicases play in DNA replication?


  They untwist the double helix at the replication forks
• What is the function of single-strand binding proteins?


  They bind to and stabilize single-stranded DNA
• What does topoisomerase do during DNA replication?


  It relieves the strain of twisting of the double helix
• What is required for DNA polymerases to synthesize a new DNA strand?


  A primer and a DNA template strand
• What is the initial nucleotide chain synthesized by primase called?


  RNA primer
• What is the rate of elongation for DNA synthesis in bacteria?


  About 500 nucleotides per second
• What is the difference between dATP and ATP?


  dATP has deoxyribose while ATP has ribose
• How does DNA polymerase elongate a new DNA strand?


  By adding nucleotides only to the free 3′ end of a growing strand
• What is the leading strand in DNA replication?


  The strand synthesized continuously toward the replication fork
• What are Okazaki fragments?


  Segments of the lagging strand synthesized away from the replication fork
• What enzyme joins Okazaki fragments together?


  DNA ligase
• What is the DNA replication machine?


  A complex of proteins that participate in DNA replication
• How do DNA polymerases proofread newly made DNA?


  By replacing any incorrect nucleotides
• What is mismatch repair in DNA?


  Repair enzymes replace incorrectly paired nucleotides
• What is nucleotide excision repair?


  A process where a nuclease cuts out and replaces damaged stretches of DNA
• What is the error rate after proofreading and repair?


  Low but not zero
• What are mutations?


  Sequence changes that may become permanent and can be passed on to the next generation
• What is the significance of telomeres in eukaryotic chromosomes?


  They postpone the erosion of genes near the ends of DNA molecules